FIELD OF THE INVENTION
The invention relates to a device for burning a fuel in air. The device includes an annular passage for directing the air in a meridional flow with regard to an axis; a swirl cascade for imposing a swirl on the flow; and a mixer for intermixing the fuel with the flow forming an essentially homogeneous mixture.
In addition, the invention relates to a method of burning the fuel in air, in which the air is provided in a flow encircling an axis and continuing meridionally with regard to the axis and with a swirl is essentially mixed homogeneously with fuel for forming a mixture, which is ignited in order to burn the fuel.
Such a device, under the designation "premix burner", is known to persons skilled and active in the relevant art, the designation already pointing to the fact that the fuel is burned only with a certain time interval after its intermixing with the air provided for the combustion. The method is also known to the persons skilled and active in the relevant art as the method that takes place during the operation of a conventional premix burner.
During the operation of a conventional premix burner, when the feeding of fuel to the burner is increased, a state is often reached in which the combustion becomes unstable and acoustic vibrations are caused in the plant into which it is fitted. The acoustic vibrations are known by the term "combustion vibrations". The combustion vibrations may be so great that they jeopardize the operation of the premix burner and the plant, of which the premix burner is an integral part. The tendency of a premix burner to form unstable combustion becomes all the greater, the more homogenous the mixture of fuel is and the air formed in the premix burner before the combustion. However, a mixture which is as homogeneous as possible is desired in view of the fact that the production of nitrous oxides during the combustion is lower, the more homogenous is the mixture. If the mixture is completely homogenous, the maximum temperature occurring during the combustion of the mixture assumes a minimum, and it is precisely this effect which is essential for an especially low production of nitrous oxides.
European Patents EP 0 193 838 B1 and EP 0 589 520 E1 also disclose such a device and method.
To stabilize the combustion of a premix burner, it has been proposed to envelope the igniting mixture flowing from the burner with a veil of air and thus prevent vortices from forming in marginal regions of the mixture, in which vortices combustion processes take place, from which it may be assumed that they contribute substantially to the destabilization of the combustion. However, a disadvantage of the proposed measure may be seen in the fact that the air which is used to envelope the mixture has to be extracted from the actual combustion operation. If the thermal output to be released by the premix burner is fixed, the quantity of fuel to be used is also essentially fixed, and a withdrawal of air for stabilizing the combustion results in the actual combustion taking place in the presence of a reduced quantity of air and, in view of the fact that the combustion, in particular in a gas-turbine plant, is effected as a rule with excess air, must proceed with a markedly increased maximum temperature and thus with a markedly increased formation of nitrous oxides.